Volatile organic compounds (VOCs) have been used for decades in many industrial processes. For example, various VOCs are frequently used as building block for chemical syntheses, as solvents, or lubricants. In still other uses of particular VOCs, MTBE is used as fuel oxygenate in gasoline to reduce carbon monoxide emissions. Annual production of MTBE in the EU was reported to about 3 million tones, with even higher annual production in the U.S.
Unfortunately, MTBE and other VOCs (e.g., benzene, toluene, ethyl benzene, xylene) have recently been found in groundwater in the U.S. and other countries as a result of direct or indirect discharge into the soil, atmosphere, and/or groundwater. Among other routes, VOCs reach the groundwater via incomplete combustion from automobile engines, surface spills or leaks of fuel, leaking underground fuel tanks and piping systems. The U.S. Environmental Protection Agency has tentatively classified MTBE as a possible human carcinogen, and the California EPA Office of Environmental Health Hazard Assessment (OEHHA) established an interim action level of 35 ppb for MTBE. The importance of MTBE removal is further compounded by the fact that MTBE is relatively inert to biodegradation in most soils and/or aquifers. Presently, it is estimated that over 1600 groundwater wells in more than 39 states in the U.S. have substantial levels of MTBE contamination, and additional wells with significant MTBE levels have recently been reported in Canada.
There are numerous configurations and methods for VOC removal and/or destruction known in the art. For example, VOCs can be selectively removed from a source stream using membrane filtration as described in U.S. Pat. No. 5,954,966 to Matsuura et al. However, such systems often fail to provide an economically attractive solution, as membrane manufacture is relatively complex. Furthermore, and especially compared with air stripping, the filtration rate using such membranes is relatively low. Alternatively, VOCs may be removed from a source using adsorption to a solid material, and typical processes are described in WO 2003/062153 to Mirzayi et al. (using activated charcoal), U.S. Pat. No. 5,814,132 to Grime et al. (using coated aluminosilicates), or European App. No. 0822004 (using an organic polymer). While such solid adsorbents are generally inexpensive, simple to use, and provide relatively high flow rates, various disadvantages remain. Among other things, various VOCs (and especially MTBE) adsorb only poorly to activated charcoal. Moreover, the sorbent typically requires separate regeneration or disposal, thus shifting the MTBE problem merely to another locale.
Similarly, solvents can be employed to remove VOCs from a source material, and numerous solvent-based processes are described in the art. For example, U.S. patent application Ser. No. 2003/0094099 to Lin et al. employs water as a solvent, and the VOCs are then treated with ozone or peroxide. Alternatively, as described in U.S. Pat. No. 6,165,253 to Sirkar et al., a paraffinic oil or synthetic hydrocarbon solvent is employed to absorb the VOC, while Grasso et al. teach in U.S. Pat. No. 5,198,000, use of motor oil, mineral oil, or corn oil as an absorbent. However, the energy costs of most solvent processes are relatively high due to the circulation pumps and solvent heaters in the regeneration process.
In further known processes, air stripping is employed to drive the VOC from water into a gas phase. Air stripping can be performed in situ in soil as described in U.S. patent application Ser. No. 2002/0144953 to Kerfoot, using micro bubbles, or in U.S. Pat. Nos. 5,389,267 and 5,180,503 to Gorelick, using gas lift pumping. Air stripping may also be performed in stripping vessels or columns as described in U.S. Pat. No. 5,171,334 to Kabis. While air stripping is a relatively simple process, the liberated MTBE must be captured and destroyed, which is often problematic in in situ applications.
Once isolated from a source (e.g., air or water), the VOCs can be destroyed using various methods well known in the art. For example, in one approach, MTBE and other VOCs are transformed into less toxic compounds using microorganisms as described in U.S. Pat. No. 6,365,397 to Salanitro, U.S. Pat. No. 6,194,197 to Hyman et al., U.S. Pat. No. 5,714,379 to Phipps Jr., or in U.S. patent application Ser. No. 2003/0129735 to Moorhead. Biological transformation of VOCs is often desirable, as little or no equipment is needed to clean up a contaminated site. However, biotransformations are often relatively slow. Moreover, where relatively toxic additional contaminants are present, microorganisms may not be viable for a sufficient time to degrade the VOC to the desired extent.
In another approach, MTBE and other VOCs can be photolytically destroyed as described in U.S. Pat. Nos. 6,117,335 and 6,200,466 to Bender using near blackbody radiation. In further examples of photolytic destruction, UV irradiation and supplemental ultrasound energy is used to destroy VOCs as taught by Sato in U.S. Pat. No. 6,617,588. While photolytic destruction is often relatively effective, various disadvantages remain. Among other problems, large quantities of water are often impractical to treat as water absorbs UV relatively strongly. Furthermore, to ensure substantially complete VOC destruction, the relatively high-energy demand often renders such technology uneconomical.
In a still further approach, VOCs may be electrolytically destroyed either via direct oxidation at the anode, or indirectly via generation of oxidizing species in the electrolytic cell. For example, Cole teaches in U.S. Pat. No. 5,531,865 the use of sacrificial electrodes, while Breault teaches in U.S. Pat. No. 5,637,198 use of corona discharge. Such methods are generally effective in VOC destruction, however, suffer from similar problems as photolytic methods. Most significantly, the energy demand is often undesirably high. To circumvent at least some of the problems associated with energy demand, VOCs can be contacted with oxidizing species as taught by Wasinger in U.S. Pat. No. 6,197,206 or U.S. patent application Ser. No. 2002/0144953 to Kerfoot. In further known oxidative processes, molten yellow phosphorus may be employed as an oxidant as described in U.S. Pat. No. 5,332,563. Such processes are conceptually simple, however, typically require exact dosing or post-treatment steps where over-oxidation or residual oxidant in the water is undesirable.
In most preferred methods, MTBE and other VOCs are catalytically oxidized using various catalysts. For example, Taylor et al. use uranium oxide to catalytically destroy various VOCs. In other examples, as described in U.S. Pat. No. 6,193,504 to Chen, U.S. Pat. No. 5,914,091 to Holst et al., or U.S. Pat. No. 5,609,829 to Lucas et al., platinum, manganese, or copper are used on a ceramic material to form a catalyst that assists in VOC oxidation. Suitable catalysts may also be admixed with a VOC adsorbent as taught by Campbell et al. in U.S. Pat. No. 6,479,022. While the use of catalysts typically significantly reduces energy requirements as compared to many electrolytic or photolytic methods, most of the known catalysts still require relatively high temperature for proper operation. To reduce temperature requirements, mixed metal oxide or multi-phase catalysts can be employed as described in WO 01/45833 or U.S. Pat. No. 6,458,741 to Roark et al., or in U.S. Pat. Nos. 5,851,948 and 5,009,872 to Chuang et al.
Such low-temperature catalyst in combination with air stripping typically provides technologically simple and effective systems for VOC removal and destruction. For example, in U.S. Pat. No. 5,190,668 to Chuang, the inventor describes a process in which VOCs are stripped from a liquid and oxidized using a noble metal oxidation catalyst that is deposited on a hydrophobic support. A similar system using certain precious metal catalysts is described in U.S. Pat. No. 4,892,664 to Miller, and further suitable catalysts are described. Such systems generally provide various advantages over other systems described above. However, energy consumption and flow rate are often still less than desirable. Moreover, and particularly where noble metals are employed, the cost of such catalysts frequently reduces the economic attractiveness.
Therefore, while numerous configurations and processes for water purification are known in the art, all or almost all of them, suffer from one or more disadvantages. Thus, there is still a need for improved water purification plants and methods, and especially for removal and destruction of MTBE from water.